Born in Brooklyn, New York, Susan Maria Smith was the first black woman to graduate from medical college in New York state. After graduating as valedictorian from the New York Medical College for Women, she attended the Long Island Medical College Hospital, where she was the only woman in the entire college.

Married in 1874 to the Rev. William G. McKinney, she practiced as Dr. Susan Smith McKinney until his death in 1896.

Smith McKinney was a cofounder, in 1881, of the Women's Hospital & Dispensary in Brooklyn, which later became the Memorial Hospital for Women and Children. She served on the staff of the New York Medical College and Hospital for Women in Manhattan, and from 1892 to 1896 was manager of the medical staff of the Brooklyn Home for Aged Colored People. She also served as church organist and choir director for Brooklyn's Bridge Street Church.

In 1896, she married Chaplain Theophilus Gould Steward and became resident physician at Wilberforce University in Ohio.

In 1911 she addressed an interracial congress in London on "Colored Women in America." She read a paper on "Women in Medicine" before the National Association of Colored Women's Clubs at Wilberforce, Ohio, in 1914. This paper includes a list of black women who pioneered in medicine.

In 1975, the Dr. Susan Smith McKinney Junior High School in Brooklyn was named to commemorate her accomplishments.